The present invention relates to unstable laser resonators and particularly to an unstable laser resonator which produces optical radiation that simultaneously has the high output power and diffraction-limited divergence that are characteristic of an unstable laser resonator and also the narrow bandwidth that can usually be obtained only with a stable laser resonator.
Many applications of lasers require that they produce radiation that simultaneously possesses the characteristics of high power, diffraction-limited divergence and narrow-bandwidth. In addition, a symmetric output profile is desirable in many situations for purposes of optimal propagation or focusing. Various techniques have been developed for producing laser radiation with each of these characteristics. However, in many situations the requirements for producing one of these characteristics conflicts with those for producing the others, with the result that systems that simultaneously possess all three characteristics are usually expensive and complicated, often consisting of multiple lasers. The following techniques are currently available for producing laser radiation with two of these characteristics.
A stable laser resonator can be used to produce laser radiation that has low spatial divergence and narrow-bandwidth. The low spatial divergence is obtained by restricting the laser to operate in the lowest transverse mode of the stable laser resonator cavity. For wavelengths in the near infrared, or shorter, this usually requires cavity apertures with diameters on the order of one millimeter (1 mm) or less. As a result, the power available from a stable laser resonator is limited, often to values that are orders of magnitude less than could be obtained if larger apertures (which are available) were used. The bandwidth of a stable laser resonator is usually restricted by using a dispersing element in the resonator or cavity, (e.g. a grating, prism, Fabry-Perot etalon, or some combination of these) to spread the radiation in angle according to wavelength. Narrow-bandwidth operation is then obtained by restricting the angular acceptance of the cavity. This operation is compatible with the mode-restricting aperture used for limiting the spatial divergence that was described above, but is again incompatible with high power operation.
High power can be obtained with narrow-bandwidth, low divergence laser radiation by the use of laser amplifiers following the stable laser resonator. Such a resultant system is currently the only known prior art method for obtaining laser radiation with all three of these characteristics. While being capable of producing radiation with the desired characteristics, the multiple lasers that are required increase the size and cost of the system and the complexity of the optical train required to match the laser beam into each of the laser amplifier stages. In many cases the reliability of pulsed systems can be degraded because of jitter in timing between the various laser amplifier stages.
An unstable laser resonator or cavity is an alternative approach to obtaining high power, low divergence laser radiation. In an unstable laser resonator the laser radiation can be made to fill a relatively large diameter cavity to allow operation at high power levels, while the dynamics of the unstable resonator restrict the divergence of the output laser radiation to a low value, usually near the diffraction limit for a suitably designed system. However, such a prior art unstable laser resonator has a broad-bandwidth.
Some success has been achieved in the frequency narrowing of the laser radiation from an unstable laser resonator cavity by using a diffraction grating. This technique has worked best with lasers that have sharp line structure, such as molecular lasers. For example, selection of a single line in a hydrogen-fluoride laser has been reported in several configurations involving the insertion of a diffraction grating into a standard unstable laser resonator cavity. However, with other types of lasers that have a broad band continuous gain distribution, such as excimer lasers or dye lasers, the simple insertion of a diffraction grating in a standard unstable laser resonator configuration does not provide sufficient spectral discrimination for narrow band operation. This result is due to the fact that the standard unstable laser resonator cavity is fundamentally incompatible with the needs of frequency narrowing elements used to restrict the bandwidth, especially when extremely narrow linewidths are desired. The nature of the modes in an unstable laser resonator requires that the divergence of the laser radiation inside the cavity alternate between low and high values on alternate passes through the cavity. As a result, it becomes impossible to use angular discrimination to restrict the bandwidth of the laser radiation as was done in stable laser resonator cavities. Thus, although currently available unstable laser resonators have the configuration of choice for producing high power, low divergence radiation from laser cavities, they are not compatible with a simultaneous requirement of narrow-bandwidth.